# LeetCode – Combination Sum III (Java)

Find all possible combinations of k numbers that add up to a number n, given that only numbers from 1 to 9 can be used and each combination should be a unique set of numbers.

Ensure that numbers within the set are sorted in ascending order.

Example 1: Input: k = 3, n = 7 Output: [[1,2,4]]
Example 2: Input: k = 3, n = 9 Output: [[1,2,6], [1,3,5], [2,3,4]]

Analysis

Related problems: Combination Sum, Combination Sum II.

Java Solution

```public List<List<Integer>> combinationSum3(int k, int n) { List<List<Integer>> result = new ArrayList<List<Integer>>(); List<Integer> curr = new ArrayList<Integer>(); helper(result, curr, k, 1, n); return result; }   public void helper(List<List<Integer>> result, List<Integer> curr, int k, int start, int sum){ if(sum<0){ return; }   if(sum==0 && curr.size()==k){ result.add(new ArrayList<Integer>(curr)); return; }   for(int i=start; i<=9; i++){ curr.add(i); helper(result, curr, k, i+1, sum-i); curr.remove(curr.size()-1); } }```

Note the following relation in Java:

Integer is a subclass of Number and ArrayList is a subclass of List. But ArrayList is not a subclass of ArrayList.

Category >> Algorithms >> Interview
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• Suzy

Thank you for the great solution!

• Nilesh Barge

What is time complexity of this solution?